In accordance with the miniaturization of, for example, electronic equipment such as a portable telephone or a notebook-sized personal computer, the demand for further miniaturization and weight reduction of a secondary battery as a power source thereof is increasing year after year, and the enhancement of the performance of the secondary battery is also being demanded. In particular, the secondary battery in which an alkali metal, especially lithium, is used as an active material of a negative electrode has generally various advantages. For example, it not only ensures high energy density and high electromotive force, but also has wide operating temperature range due to the use of a nonaqueous electrolyte. Further, the secondary battery is excellent in shelf life, miniaturized and lightweight.
However, when metallic lithium is used in a negative electrode for lithium-ion secondary battery, the problem has been encountered such that the repetition of charge and discharge of the secondary battery occasionally causes formation of dendrites on the surface of the metallic lithium and the growth of dendrites causes the piercing of the separator disposed between the positive and negative electrodes so that the dendrites contact the positive electrode to thereby invite a short-circuit.
In recent years, it has been demonstrated that the above formation of dendrites can be prevented by using a carbon based material (carbon material or graphite material) in which lithium ions are occluded for the negative electrode. Thus, energetic studies are being made on the carbon based material as the most promising material for the negative electrode of secondary battery.
The carbon based material is generally classified into the carbon material and the graphite material, depending on the level of growth of graphite texture.
The carbon materials having been employed as a material of a negative electrode for a lithium-ion secondary battery include not only natural materials such as coal, but also coal-based carbon materials such as coke, polymeric compound-based carbon fibers such as polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fibers which are not graphitized, and pitch-based spherical carbon materials. These carbon materials are each composed mainly of amorphous carbon, contain small graphite crystallites, and have a disorderly crystal arrangement, so that a negative electrode having larger charge and discharge capacities than those of the graphite materials can occasionally be obtained, depending on sintering temperature. Further, the lithium-ion secondary battery including the above negative electrode exhibits a slow potential change upon completion of discharge, so that the residual capacity can be displayed on the basis of this potential change. Still further, the above negative electrode is characterized by having excellent properties at low temperature as compared with those of the negative electrodes based on graphitic materials.
However, the use of the negative electrode based on the above carbon material in the secondary battery involves the problem that the charge and discharge capacities are gravely lowered by the repetition of charge and discharge cycles.
On the other hand, with respect to the graphite material, the use of artificial materials such as artificial graphite and graphite fiber and spherical graphite material obtained by graphitizing the above carbon fiber and spherical carbon material, respectively, has been proposed in addition to the natural material such as natural graphite.
In these graphite materials, the graphite crystallites grow large, the crystal arrangement has little disorder and it is believed that a lithium atom is orderly intercalated between crystallites in a lithium atom to carbon atom proportion of 1:6. The negative electrode for a lithium-ion secondary battery which is based on the above graphite material is advantageous in that the change of charge and discharge capacities by the repetition of charge and discharge cycles is slight.
The theoretical capacity based on the above lithium atom occlusion arrangement is 372 mAh/g for the negative electrode for a lithium-ion secondary battery which is based on the graphite material. However, the real capacity of the negative electrode based on the conventional graphite material has not reached this theoretical capacity and it has been difficult to realize the charge and discharge capacities which are comparable to those exhibited in the use of the carbon material.
Moreover, the negative electrode based on the graphite material involves the problem that the potential change is sharp at the termination of charge and discharge to thereby render a display of residual capacity difficult.
Various studies and investigations have been made with a view toward development of a carbon-based material for a negative electrode for secondary battery combining the properties of the carbon material such that, when used in a negative electrode for a secondary lithium battery, the charge and discharge capacities are large, excellent low temperature performance is ensured, and residual capacity can be displayed, with the properties of the graphite material such that the cycle deterioration of charge and discharge capacities is slight.
For example, Japanese Patent Laid-Open Publication No. 1(1989)-221859 proposed heating pulverized coke in an inert gas stream or in vacuum at a temperature at which no graphitization occurs. However, the inventors have studied the above heating of pulverized coke in an inert gas stream or in vacuum and have found that, although the heating must be conducted at a relatively high temperature for attaining a substantial improvement in cycle deterioration, the high capacity characteristics per se inherently possessed by the carbon material are gravely deteriorated thereby.
Japanese Patent Laid-Open Publication No. 7(1995)-335263 proposed a process comprising preparing a paste from graphite powder coated with a metal such as Ni or Cu, coating a metal plate of, for example, titanium with this paste and drying to thereby obtain a negative electrode for a secondary lithium battery in order to control the capacity deterioration. However, the addition of a metal to graphite powder for attaining an improvement in capacity cycle deterioration involves the problem that the weight of the negative electrode increases to thereby cause a grave decrease of the battery capacity per weight and, thus, renders its practicability poor.
Negative electrodes for secondary lithium batteries containing milled graphite fibers which are prepared by spinning a mesophase pitch, optionally lightly carbonizing the resultant pitch fiber, milling the pitch fiber, and carbonizing and graphitizing the milled pitch fibers, are disclosed in Japanese Patent Laid-Open Publication Nos. 7(1995)-85862, 8(1996)-69798, 9(1997)-63584 and 9(1997)-63585. These negative electrodes have such properties that the charge and discharge can be performed at high current density, the charge and discharge capacities are high and the electrolyte is scarcely decomposed at the time of charge and discharge. However, when the above milled graphite fibers are used singly, the potential change is sharp at the termination of charge and discharge and a negative electrode having charge and discharge capacities which are equal to or greater than those exhibited in the use of the carbon material has not yet been realized.
Thus, mixings of carbon materials, graphite materials and carbon materials with graphite materials have been investigated and studied in order to compensate each other's drawbacks.
For example, Japanese Patent Laid-Open Publication No. 6(1994)-111818 discloses that an electrode sheet (negative electrode) obtained by mixing spherical graphite particles with short graphitized carbon fibers (obtained by graphitizing carbon fiber grown in the vapor phase) in an appropriate proportion exhibits an enhanced conductivity to thereby enable realizing a high capacity and also exhibits an enhanced electrode strength and enables preventing carbon material falling and falling from a collector base with the result that the cycle life can be prolonged. However, it has been found that the discharge capacity is decreased and the mixing effect is unsatisfactory, depending on conditions.
Japanese Patent Laid-Open Publication No. 5(1993)-283061 discloses that a combination use of carbon particles with carbon fiber in a negative electrode makes the conductivity increased and realizes a bulky structure to thereby increase the diffusion of electrolyte through pores, so that a secondary lithium battery which is excellent in charge and discharge velocities, output density, and cycle characteristics can be obtained. However, it has been found that the discharge capacity is unsatisfactorily as small as 270 mAh/g.
Japanese Patent Laid-Open Publication No. 3(1991)-129664 discloses the use in a negative electrode of a composite material comprising fine fibrous graphite and, borne between fibers thereof, a carbonaceous material made from organic polymeric material enables increasing the packing density of the electrode, improving the voltage flatness at discharge or the charge and discharge cycle characteristics, and increasing the energy density. However, it has been found that the initial charge and discharge efficiency is as low as 67% and that the amount of lithium inactivated at the first cycle is large, thereby to render its practicability poor.
Japanese Patent Laid-Open Publication No. 6(1994)-150931 discloses a process comprising mixing an amorphous particulate graphite material with a carbon material of pitch-based carbon fiber in an attempt to enhance the conductivity which is a drawback of the carbon material, increase the charge and discharge velocities which are a drawback of the graphite material, and improve the cycle characteristics. However, the discharge capacity is still as small as about 200 mAh/g.
Japanese Patent Laid-Open Publication No. 7(1995)-161347 discloses a process comprising mixing in equal amounts a highly crystalline PAN-based carbon fiber with low resistivity which has been carbonized at high temperature with a lowly crystalline PAN-based carbon fiber with high resistivity which has been carbonized at low temperature to thereby compensate each other's drawbacks and realize a negative electrode material having a large discharge capacity and exhibiting a low initial capacity loss. However, it has been found that the initial discharge capacity is still as low as about 240 mAh/g and the initial charge and discharge efficiency is still as low as about 55%, so that the obtained material cannot serve practical use.
Japanese Patent Laid-Open Publication No. 7(1995)-192724 discloses that a composite (mixture) of a natural or synthetic powdery graphite with a powdery carbon material such as a carbon material whose graphitization is difficult and/or a carbon material whose graphitization is easy has both of the high true density of graphite and the capability of diffusing lithium ions at high velocity of the carbon material and has such characteristics that the charge and discharge performance is high and the stability of positive electrode is not deteriorated. However, the powdery graphite used as a negative electrode material therein is natural graphite or an artificial graphite obtained by carbonizing an organic material and heating the carbonization product at high temperature, and a battery performance as negative electrode material in the composite of the above graphite with the powdery carbon material is exhibited by a special operation of intermittent charge and discharge which is not a common technique.
The inventors have made various investigations and studies with the intent to solve the above problems of the prior art. As a result, it has been found that a novel carbon material comprising a carbonaceous internal part and a graphitized surface can be prepared by heating a carbon material in the presence of a specified metal compound at specified temperature and that the use of this carbon material as a negative electrode enables obtaining a secondary battery exhibiting large charge and discharge capacities, being excellent in low temperature performance, facilitating a display of residual capacity, and ensuring excellent cycle characteristics of charge and discharge capacities. The present invention has been completed on the basis of the above findings.